Method of measuring flexure value of wire electrode

ABSTRACT

A method of automatically measuring the flexure value of a wire electrode 1 in a wire-cut electric discharge machine is disclosed. 
     At a measurement point in the course of electric discharge machining, the electric discharge is temporarily stopped, and whether or not the wire electrode 1 and a workpiece 2 come into contact is sensed by a contact sensing device 107a. The wire electrode 1 and the workpiece 2 are moved relatively to advance or retreat in accordance with the sensed touch signal TS, and the distance ε 1  of movement until the contact state changes, is measured. The flexure value ε is obtained calculatively from the measured movement distance ε 1 .

This is a continuation of co-pending application Ser. No. 413,375 filedon Aug. 25, 1982 now U.S. Pat. No. 4,521,662.

TECHNICAL FIELD

The present invention relates to a method of automatically measuring thevalue of flexure of a wire electrode due to electric discharge duringelectric discharge machining in a wire-cut electric discharge machine.More particularly, it relates to a method of automatically measuring thevalue of flexure of a wire electrode by utilizing the constituents of awire-cut electric discharge machine.

BACKGROUND ART

The operating principle of a wire-cut electric discharge machine is thata predetermined gap is maintained between a wire electrode and aworkpiece, while a voltage is applied therebetween to cause the sparkdischarge across the gap, whereby the workpiece is cut by the dischargeenergy. Accordingly, when the workpiece is moved relative to the wire onthe basis of machining command data, it can be machined into a desiredshape. In such a wire-cut electric discharge machine, when the wireelectrode 1 advances in a groove 3 in the workpiece 2 in a predetermineddirection while cutting the workpiece 2 by virtue of the electricdischarge, as shown in an operation explaining diagram of FIG. 1, apressure attributed to the electric discharge develops between the wireelectrode 1 and the workpiece 2 as illustrated in a sectional view ofFIG. 2, with the result that the wire electrode 1 is pushed back in thedirection of an arrow, namely, in the direction opposite to theadvancing direction. Therefore, the wire electrode 1 lies behind theposition of wire guides 4, 4. That is, the wire electrode 1 flexes. Whenperforming the electric discharge machining of a straight groove, theflexure does not affect the machining accuracy and is not a seriousproblem.

In a machining operation for forming a corner part, however, the flexurebecomes an important problem. In order to form a groove 3 which consistsof a first straight groove L₁ and a second straight groove L₂ orthogonalto the former, as shown in a front view of the machined groove in FIG.3, a corner part CN needs to be formed at the intersection point of thefirst and second straight grooves L₁ and L₂ by machining. In forming thecorner part CN, after the first straight groove L₁ has been formed bythe unidirectional relative movement between the workpiece 2 and thewire electrode 1, this relative movement is changed into the orthogonaldirection by a machining command. On account of the foregoing flexure ofthe wire electrode 1 ascribable to the electric discharge, however, thewire electrode 1 of the discharging part is dragged inwardly of thecorner part CN, and unlike the commanded shape (indicated by a solidline), the machining path of the groove 3 deviates on the inner sideconsiderably as indicated by dotted lines, so that the machined shapebecomes blunt.

Likewise, when the corner part CN' between the first straight groove L₁and the second straight groove L₂ is to be machined in the shape of acircular arc as shown in a front view of the machined groove in FIG. 4,the machining path of the corner part CN' becomes a blunt machinedshape, indicated by the dotted lines, in contrast to the commanded shapeof a solid line, because of the flexure of the wire electrode 1ascribable to the electric discharge. This necessitates such acountermeasure in which the flexure value of the wire electrode ismeasured in advance, and in case of machining the corner, the workpieceis moved to a somewhat greater extent in correspondence with the flexurevalue.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a methodof measuring the flexure value of a wire electrode which canautomatically measure the flexure value of the wire electrode.

Another object of the present invention is to provide a method ofmeasuring the flexure value of a wire electrode which can measure theflexure value of the wire electrode by utilizing the construction of awire-cut electric discharge machine.

Further, another object of the present invention is to provide a methodof measuring the flexure value of a wire electrode which permitsmachining to be restarted after the measurement of the flexure value ofthe wire electrode.

The present invention comprises a detection step of temporarily stoppingelectric discharge at a predetermined measurement point during electricdischarge machining and then detecting the contact between a workpieceand a wire electrode at that time by detection means; a distancemeasuring step of advancing or retracting the wire electrode relative tothe workpiece, depending upon whether or not the wire electrode lies incontact with the workpiece, thereby to measure a distance up to theplace where the wire electrode and the workpiece comes into or out ofcontact; and a calculation step of calculating the flexure value of thewire electrode in accordance with the measured distance.

According to the present invention, therefore, not only the flexurevalue of the wire electrode can be automatically measured, but also suchmeasurement can be executed by utilizing the existing construction of awire-cut electric discharge machine, and the machining can be readilyrestarted after the measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the principle of the wire-cut electric dischargemachining to which the present invention is applied;

FIG. 2 is a sectional view of the workpiece illustrating the flexure ofa wire electrode to which the present invention is directed;

FIGS. 3 and 4 both illustrate machining errors attributed to theflexure;

FIG. 5 illustrates a method of measuring a flexure value according tothe present invention;

FIG. 6 is a block diagram of an embodiment of the present invention; and

FIG. 7 is a circuit diagram of a contact sensing device used in theembodiment of FIG. 6.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will now be described in detailwith reference to the drawings.

FIG. 5 illustrates a method of measuring a flexure value according tothe present invention.

In the figure, symbols 1 and 1' both denote the sections of a wireelectrode. The circle indicated at 1 is a wire electrode position at themaximum flexure value, while the circle indicated at 1' is a wireelectrode position at a guide position. Numeral 2 designates aworkpiece, and numeral 3 a groove formed by the electric dischargemachining.

In the present invention, the measurement of a flexure value isperformed in the following sequence:

(a) In the course of machining, the electric discharge is stoppedtemporarily at a predetermined measurement point (FIG. 5). Since thecessation of the electric discharge results in nullifying the pressureof the electric discharge, the wire electrode 1 having flexed is drawntoward guides (in a direction A in the figure) to come into contact withthe workpiece 2.

When the wire electrode 1 has come into contact with the workpiece 2, acontact sensing device to be described later senses this contact.

(b) From the state in which the wire electrode 1 lies in contact withthe workpiece 2, the wire electrode 1 is retracted relative to theworkpiece 2 along the machining path. In retraction control, a table onwhich the workpiece 2 is placed may be moved relative to the wire 1 inthe direction of the arrow A, or the guides may be retracted relative tothe workpiece in the direction of an arrow B in a wire-cut electricdischarge machine of the construction in which the guides can be moved.

(c) As the retraction control is continued, the contact between the wireelectrode and the workpiece is released. The retraction distance ε₁ fromthe discharge stopping position to the position where the contactbetween the wire electrode and the workpiece is released is measured inthe aforementioned retraction control, and is stored in a memory builtin an NC (numerical control device).

(d) When a true flexure value ε is subsequently calculated by the NC inaccordance with the following equation, the measurement of the flexurevalue ends:

    ε=g+ε.sub.1                                (1)

Here, g denotes the gap of the electric discharge, and it is obtainedfrom the following equation when the width of the machined groove isindicated by l and the diameter of the wire by φ as shown in FIG. 5:

    g=(l-φ)/2                                              (2)

Accordingly, when the machined groove width l and the wire diameter φare measured and entered into the NC in advance, the flexure value ε isfound by executing the calculations of Equations (1) and (2). While, tothe end of evaluating the discharging gap g, l and φ may be previouslymeasured and entered in this manner, the discharging gap g may beactually measured by carrying out processing to be described below. Whenthe contact between the wire electrode 1 and the workpiece 2 has beenreleased by the foregoing retraction control, the wire electrode ismoved in the direction orthogonal to the machining direction (machiningpath), so as to measure the distance to a position where the wireelectrode and the workpiece come into contact again. Using this distanceas the discharging gap g, the flexure value ε is obtained from Equation(1).

Under some machining conditions, the wire electrode 1 and the workpiece2 do not come into contact when the electric discharge has been stoppedat the measurement point. This is because the aforementioned discharginggap g between the wire electrode 1 and the workpiece 2 is greater thanthe flexure value of the wire electrode 1. In such a case, in otherwords, in a case where the contact sensing device does not sense thecontact between the wire electrode and the workpiece after the stop ofthe electric discharge, (b') the wire electrode 1 is advanced in themachining direction until it comes into contact with the workpiece 2,(c') the advance distance ε'₁ is measured, and (d') the calculation of:

    ε=g-ε'.sub.1                               (3)

is executed, whereby the flexure value ε is obtained.

The above is the outline of the method of measuring the flexure valueaccording to the present invention. Further, the present invention is soimproved as to permit the measurement of the flexure value with highprecision. More specifically, when there are machining scraps in themachined groove 3, some measurement error may occur in the foregoingmethod. In the actual measurement of the flexure value, therefore,processing steps to be described below are added. In this way, themeasurement of the flexure value with higher precision is permitted.That is, (e) when the wire electrode 1 has reached the measurementpoint, or when the flexure value measurement command has been generated,the wire electrode 1 is retracted to a predetermined position relativeto the workpiece, whereupon (f) while performing the electric discharge(re-machining the workpiece), the wire electrode is advanced quickly tothe measurement point again and then positioned at the measurementpoint, and after the electric discharge has been stopped, the flexurevalue measurement processing is executed by the foregoing steps (a) to(d). Due to the re-discharging (re-machining) processing, the machiningscraps are blown away, so that a flexure value measurement of highaccuracy is realized. Moreover, since the workpiece is re-machined athigh speed, it is not excessively cut, and the flexure value does notchange. Ordinarily, the re-machining speed needs to be at least 10 timeshigher than the normal machining speed in order to prevent the flexurevalue from changing. Besides, the retraction before the re-machiningstage may be done at high speed while the electric discharge is beingperformed, or it may be done until the short-circuit is released,without performing the electric discharge. Further, the machiningconditions should be relaxed during the re-discharging stage, ifpossible. In addition, although the re-machining stage is allowed to berepeated, too many re-machining stages results in cutting the workpieceexcessively and creating a measurement error.

The measurement processing described above is carried out by entering anauxiliary command such as machining program "M20" before machining theworkpiece and storing a measurement processing program in a memory for acontrol program, built in the NC, beforehand. That is, when the "M20"instruction has been read out, the measurement processing program startsto execute the flexure value measurement processing.

FIG. 6 is a block diagram of an embodiment for realizing the method ofmeasuring the flexure value according to the present invention.

Referring to the figure, numeral 101 designates an NC (numerical controldevice) constructed of a microcomputer. It includes a memory 101a whichstores the processing program for measuring the flexture value and thecontrol program for executing the other numerical control processing, aprocessing unit 101b which executes predetermined processing on thebasis of commands from the control program and a machining program to bedescribed below, a data memory 101c which stores data and othercalculated results, a tape reader 101d, etc. Numeral 102 designates amachining program tape storing machining data such as electric dischargemachining paths and machining speeds. The auxiliary functioninstructions "M20" for the flexure value measurement are entered insuitable places of the machining program. Numerals 103 and 104 indicatepulse distributors which execute predetermined pulse distributioncalculations on the basis of movement value commands X_(c) and Y_(c)issued from the NC, so as to provide distributed pulses X_(p) and Y_(p),respectively. Numerals 105 and 106 indicate moving directioncontrollers, which change the signs of the distributed pulses and thenprovide the resulting pulses in the retraction control. Shown at numeral107 is a wire-cut electric discharge machine which comprises a wireelectrode; a wire guide mechanism for supporting the wire electrode; atable for placing a workpiece thereon; a pair of X-axial and Y-axialmotors for moving the table in the directions of X- and Y-axesrespectively; a pair of servo circuits for driving the X- and Y-axialmotors in accordance with the given distributed pulses X_(p) and Y_(p)respectively; and an electric discharge circuit for causing an electricdischarge by applying a voltage between the wire electrode and theworkpiece. The wire-cut electric discharge machine 107 will not bedescribed in greater detail because it has the well-known constructionwherein the table is moved by the motors in accordance with thedistributed pulses X_(p) and Y_(p) and an instruction DSS from the NS,thereby subjecting the workpiece on the table to an electric dischargemachining operation to cut it into a desired shape by means of the wireelectrode. Further, this wire-cut electric discharge machine 107 isprovided with a contact sensing device 107a which detects the contactbetween the wire and the workpiece.

FIG. 7 is a circuit diagram of the contact sensing device 107a, in whichWIR denotes a wire electrode, WK a workpiece, PS a machining powersupply, COM a comparator, R₁ -R₄ resistors Tr a transistor for applyingthe machining voltage between the wire and the workpiece, DD a diode,and V_(L) a detection level. Now, when the wire electrode WIR and theworkpiece WK are not short-circuited, a voltage -V_(o) is applied to thecomparator COM, and hence, the comparator COM is not enabled. Incontrast, when they are short-circuited, the potential at point Abecomes zero volt, and the comparator COM is enabled to provide acontact signal TS. Thus, the short-circuit is detected.

The transistor Tr, the resistor R₁ and the machining power supply PSconstitute the electric discharge circuit, while the diode DD, theresistors R₂, R₃ and R₄ and the comparator COM constitute the contactsensing device.

Next, operations in FIG. 6 will be briefly explained. It is supposedthat, when the electric discharge is stopped at the measurement point,the wire electrode comes into contact with the workpiece.

Ordinarily, the NC executes numerical control processing on the basis ofmachining data read out from the machining program tape 102. Morespecifically, when position control data are read out from the machiningprogram tape 102, the processing unit 101b stores the position commanddata X_(c), Y_(c) and signs SX, SY in position command registers XCR,YCR and sign registers XSR, YSR, and deliver them to the pulsedistributors 103, 104 and the moving direction controllers 105, 106,respectively. In the position command registers and the sign registers,the latest position command data and signs are stored. Upon receivingthe data X_(c), Y_(c), the pulse distributors 103, 104 execute the pulsedistribution calculations on the basis of these data X_(c), Y_(c) andprovide the distributed pulses X_(p), Y_(p). These distributed pulsesX_(p), Y_(p) are respectively supplied through the moving directioncontrollers 105, 106 to the aforementioned X-axial and Y-axial servocircuits, not shown, within the wire-cut electric discharge machine 107and thus drive the motors so as to move the table as programmed.Further, the distributed pulses X_(p), Y_(p) are entered into the NC andare counted by the processing unit 101b in accordance with the movingdirections thereof, with respect to the contents of actual positionregisters XAR, YAR within the data memory 101c, whereupon the countvalues are set in the actual position registers XAR, YAR. That is, theactual position of the table is stored in the actual position registersXAR, YAR. When the table carrying the workpiece thereon has been movedby the above control, the spark discharge occurs between the wireelectrode and the workpiece, and the workpiece is machined by theelectric discharge machining as commanded. Under this state, (1) whenthe auxiliary function instruction "M20" for the measurement of aflexure value has been read out from the machining program tape 102, theprocessing unit 101b of the NC delivers the discharge stop command DSSto the electric discharge machine 107 through a line l₁, to turn "off"the transistor Tr (FIG. 7) and to stop the electric discharge. (2)Thereafter, the processing unit 101b reads out the latest positioncommand data X_(c), Y_(c) and signs SX, SY stored in the positioncommand registers XCR, YCR and sign registers XSR, YSR, and it appliesthe position command data X_(c), Y_(c) to the pulse distributors 103,104, while it inverts the signs SX, SY and applies the resulting signsto the moving direction controllers 105, 106. In parallel with this, theprocessing unit 101b saves the contents of the actual position registersXAR, YAR (the positional coordinates of the measurement point) inX-axial and Y-axial saving registers XASR, YASR. On the basis of theposition command data X_(c), Y_(c), the pulse distributors 103, 104perform the pulse distributions to provide the distributed pulses X_(p),Y_(p). These distributed pulses X_(p), Y_(p) are converted into pulses,in the directions opposite those generated during the machiningoperation, by the moving direction controllers 105, 106 and inaccordance with the instructions of the signs SX, SY. The convertedpulses are then applied to the servo circuits. As a result, the tableretreats in a direction opposite the machining direction until the wireelectrode does not contact the workpiece. The distributed pulses X_(p),Y_(p) are applied to the NC, and they are counted in accordance with themoving directions and then stored in the actual position registers XAR,YAR by the processing unit 101b. (3) When the wire electrode has comeaway from the workpiece, this state is sensed (TS="0") and reported tothe NC 101 by the contact sensing device 107a, and the NC gives aninstruction to stop the retraction of the table. (4) Subsequently, theprocessing unit 101b of the NC causes the electric discharge machine 107to start the electric discharge, through the line l₁. It also appliesthe signs SX, SY to the moving direction controllers 105, 106 and causesthe pulse distributors to execute the pulse distribution calculationsagain so as to apply the distributed pulses X_(p), Y_(p) to the servocircuits through the moving direction controllers 105, 106, thereby toadvance the table at high speed. (5) When the processing unit 101b hassensed the agreement between the contents of the actual positionregisters XAR, YAR and the saving registers XASR, YASR, it stops theelectric discharge again and stops the advance of the table.

(6) If, at this time, the wire electrode lies in contact with theworkpiece, the touch signal TS is provided from the contact sensingdevice 107a. In response to the touch signal TS (="1"), the NC retractsthe wire relative to the workpiece by controls similar to the foregoingsteps (2) and (3) until they come out of contact (TS="0"). The pulsesX_(p), X_(p) generated at this time are counted, and their total numbersX_(n), Y_(n) are stored in flexure value registers BXR, BYR. (7) Whenthe contact has been released (TS="0"), the processing unit 101b of theNC evaluates the retraction distance ε₁ by calculating: ##EQU1## (wherek is a constant) (8) Subsequently, using the machined groove width l andthe wire diameter φ stored in the data memory 101c, the flexure value εis calculated and obtained from Equations (1) and (2). (9) Thenceforth,the wire electrode is advanced relative to the workpiece up to themeasurement point as in the steps (4) and (5). Then, the flexure valuemeasurement processing ends. While, in the above description, it isassumed that the wire comes into contact with the workpiece when theelectric discharge is stopped at the measurement point, even a casewhere they do not come into contact can be processed substantially inthe same way.

More specifically, when the wire and workpiece do not contact theforegoing steps (1)-(5) are executed. When, in the step (3), the contactsignal is read by the contact sensing device 107a immediately after theretracting movement, the signal TS is "0" because originally the wireand the workpiece are out of contact, and hence, the movement of thetable is immediately stopped. Thereafter, conversely to the foregoingstep (6), the wire is advanced relative to the workpiece until they comeinto contact (TS="0"). The pulses X_(p), Y_(p) developed at this timeare counted, and the total counts X_(n), Y_(n) are stored in the flexurevalue registers BXR, BYR. The foregoing step (7) is executed to obtainε₁ ', and further, the flexure value ε is obtained from Equations (2)and (3). Lastly, the wire electrode is retracted relative to theworkpiece up to the measurement point. Then, the measurement processingends.

Alternatively, in the flow of the foregoing steps, the step of detectingfrom the signal of the contact sensing device 107a whether or not thewire electrode lies in contact with the workpiece may be providedbetween the steps (1) and (2). This measure affords a sufficientretraction distance in the step (2) in the case where the workpiece andthe wire electrode are out of contact at the stop of the electricdischarge. In addition, while the pulse distributors and the movingdirection controllers have been explained as being disposed outside theNC, they may be disposed in the NC. Further, different methods ofretraction control may be contrived.

As set forth above, according to the present invention, the flexurevalue of a wire electrode can be automatically measured, and after themeasurement, machining can be restarted immediately and high precisionwire-cut electric discharge machining is achieved. Therefore, theinvention can enhance machining precision when applied to a wire-cutelectric discharge machine.

What is claimed is:
 1. A method of measuring a value of flexure of awire electrode, the flexure being caused by an electric discharge acrossa discharge gap, the method comprising the steps of:(a) detecting byohmic contact detection means whether or not the wire electrode comesinto contact with the workpiece when electric discharge is temporarilystopped at a predetermined measurement point during the course ofelectric discharge machining along a first direction in a machininggroove; (b) retracting said wire electrode relative to said workpiece ina second direction opposite said first direction along said machininggroove until the wire electrode comes out of ohmic contact with theworkpiece in the case where they lie in contact, a distance of theretraction being measured, or advancing said wire electrode relative tosaid workpiece in said first direction until the wire electrode comesinto ohmic contact with the workpiece in the case where they do not liein ohmic contact, a distance of the advance being measured; and (c)calculating the flexure value in accordance with the size of thedischarge gap, and the retraction distance or the advance distance.
 2. Amethod of measuring a flexure value of a wire electrode as defined byclaim 1, wherein in step (c), said flexure value is calculated inaccordance with the following where g denotes the size of the dischargegap and ε₁ denotes said retraction distance:

    ε=g+ε.sub.1

(where ε indicates said flexure value).
 3. A method of measuring aflexure value of a wire electrode as defined in claim 1, wherein in step(c), said flexure value is calculated in accordance with the followingwhere g denotes the size of the discharge gap and ε₁ ' denotes saidadvance distance:

    ε=g-ε.sub.1 '

(where ε indicates said flexure value).
 4. A method of measuring aflexure value of a wire electrode as defined in claim 1, wherein saidmethod further comprises the step of:prior to step (a), retracting saidwire electrode from said measurement point and thereafter returning thewire electrode to the workpiece while the electric discharge is beingperformed.
 5. A method of measuring a flexure value of a wire electrodeas defined in claim 1, wherein said method further comprises the stepof: after step (b) moving said wire electrode back to said measurementpoint.
 6. A method of measuring flexure of a wire electrode as definedby claim 1, wherein the discharge gap size is obtained by the following:

    εg=(l-φ)/2,

where L indicates the width of the machining groove, and φ indicates thediameter of the wire electrode.
 7. A method of measuring flexure of awire electrode as defined by claim 3, wherein the discharge gap size isobtained by the following:

    εg=(l-φ)/2,

where L indicates the width of the machining groove, and φ indicates thediameter of the wire electrode.